Compositional monitoring of fluid streams (gas or liquids) is critical for optimal control of industrial processes. Available monitors (analytical instruments) require costly maintenance and sampling to perform this function. To achieve greater process yields, competitive cost reductions, and increased product quality, more reliable and lower cost analytical instruments, monitors or preferably rugged and compact sensors, to better achieve above control objectives, are needed.
One example of a fluid stream process control objective is the blending of two individual refrigerants to achieve a set composition of a gaseous or liquid mixture of such refrigerants. Rugged, affordable, wide range, low power, and stable sensors are needed to enable measurement of small changes in the composition of such mixtures, or of small deviations in the set point of the concentration of one component of such a binary mixture.
Another example of a fluid stream can be found in a Proton Exchange Membrane fuel cell (PEMFC), which uses an electrochemical process to combine hydrogen and oxygen into water, producing electric current in the process. First, the combustible nature of hydrogen makes its detection and sensing vitally important from a safety point of view, in the air outside a PEMFC. Second, and of more relevance to the present invention, because hydrogen is the key fuel in a PEM fuel cell, the monitoring and control of the H2 concentration is needed for proper operation of a PEMFC. The need therefore also exists for a reliable and low-cost fluid mixture composition sensor for process monitoring and control in and emissions detection around PEMFCs.
In recent years, due to the advance of silicon semiconductor technology, much attention has been focussed on the use of a Pd metal-oxide-semiconductor (MOS) structure as a semiconductor hydrogen sensor. Pd metal has been used in hydrogen sensors because it has a good catalytic activity and can dissociate the hydrogen molecule absorbed to the surface into hydrogen atoms. A portion of the hydrogen atoms diffuses through the Pd metal and absorbes to the interface between the metal and the oxide layer. These hydrogen atoms, after polarization, cause a change in the Schottky barrier height between the oxide layer and the silicon semiconductor and thus the electrical properties of the device. In the early days, I. Lundstrom proposed a Pd/SiO2/Si MOS field effect transistor structure with a Pd gate [Lundstrom, M. S. Shivaraman, and C. Svensson, J. Appl. Phys. 46, 3876 (1975)]. After the hydrogen is absorbed to the Pd gate, the altered threshold voltage and terminal capacitance are used as the bases for the detection of hydrogen.
Another technique to sense hydrogen is to measure changes in the electrical resistivity of a Pd thin film [P. A. Michaels, Design, Development, and Prototype Fabrication of an Area Hydrogen Detector, Bendix Corporation, Southfield, Mi., 1964, Contract NAS8-5282]. A thin film is deposited on a substrate, usually in the form of two resistors in a Wheatstone bridge. An essentially inert, electrically insulating, hydrogen impermeable passivation layer covers at least one of the resistors, and the other resistor is left uncovered. The difference in electrical resistances of the covered resistor and the uncovered resistor is related to the hydrogen concentration in a fluid to which the sensor element is exposed.
The most popular (but not solely sensitive to) hydrogen sensor is the “catalytic combustible” or “hot wire” sensor. These sensors utilize as the detector element a Group VIIIB metal element (Ni, Pd, Pt) that is heated to catalytically oxidize the hydrogen, with the resulting change in temperature and associated resistance of the “hot wire or bead” being the measured parameter for the determination of the presence of hydrogen.
Although all of the above methods may be used to sense hydrogen in a gaseous mixture, they all are subject to specific limitations. All of the sensors described above are subject to small impurities that can cause uncontrolled drift, making the sensor unusable. Likewise, each of these sensors may be poisoned by trace quantities of SOx. Furthermore, the MOS semiconductor and catalytic combustible hydrogen sensors require O2 to operate. In O2 deficient environments or above the upper explosive limit, the oxidation process is quenched. This causes the hot element of the catalytic combustible sensor to heat less or not at all, causing the sensor to generate erroneous readings.
A number of approaches have been devised to measure the thermal conductivity (TC), of a fluid of interest. A traditional approach for the TC measurement has been via calorimetry using reversible step increases of energy fed to a thermally isolated or adiabatic system.
Further to the measurement of thermal conductivity, as will be discussed in greater detail below, very small, low power and affordable “microbridge” semiconductor chip sensors have been used in which etched semiconductor “microbridges” are used as heaters and sensors. The structure of such sensors might be similar to that of thermally isolated thin “hot-film” microanemometers for measuring flow rates. Semiconductor chip sensors of the class described above are treated in a more detailed manner in U.S. Pat. Nos. 4,478,076, 4,478,077, 4,501,144, 4,651,564, and 4,683,159, all of common assignee with the present invention.
A thermal conductivity based sensing approach that takes advantage of the very high thermal conductivity of hydrogen is stable and impervious to poisoning, but does suffer from interference by large changes in H2O and CO2.
The ability to make accurate measurements of the concentration of components in a mixture via thermal conductivity is generally dependent on the temperature at which such measurements are made, because each component has a different temperature dependence of thermal conductivity. The present invention, as discussed below, selects a measurement temperature at which the components differ the most in their respective values of thermal conductivity, within the limits imposed by heeding low-power, fluid stability and safety criteria.
It is therefore an object of the present invention to provide an improved fluid mixture composition sensor, using a microbridge structure, able to overcome the aforementioned deficiencies of the prior art fluid mixture detectors. More specifically, one object of the present invention is to provide an improved method of operating thermal conductivity sensors that results in: a hydrogen concentration measurement with reduced susceptibility to H2O and/or CO2; a more accurate component concentration measurement in fluid mixtures in general.
Other objects and advantages of the present invention will be more fully apparent from the ensuing disclosure and appended claims.